Cleaning systems for use in manufacturing semiconductor devices utilize ultrasonic energy from transducer crystals which causes a vibratory beam to be propagated into certain chemical cleaning solutions. The crystals may oscillate at an ultrasonic frequency in the range of between about 0.2 and 5 MHz. Such cleaning systems are sometimes known as "megasonic" cleaning systems. These systems are known to remove particles down to a least 0.3 micrometer in diameter from the surface of semiconductor wafers together with organic surface film, ionic impurities, and many other contaminants that may be on the surface. Such systems are useful to clean semiconductor wafers at all processing steps as well as cleaning ceramics and photomasks. They may also be used for photoresist removal, dewaxing and deglazing by using a suitable solvent or cleaning fluid as well as different types of stripping solutions. See U.S. Pat. No. 3,893,869 issued on July 8, 1975 to Alfred Mayer and Stanley Shwartzman for a description of such a cleaning apparatus. See also U.S. Pat. No. 4,118,649 issued on Oct. 3, 1978 to Stanley Shwartzman and Alfred Mayer for a particular form of transducer assembly useful in such apparatus.
In such systems in which the transducers are mounted on a membrane of conductive material, such as a tantalum or zirconium foil typically 5 to 50 micrometers thick, a pinhole or other rupture in the foil can develop and be the site of an electrical arc produced by the high voltages (approximately 1000 volts) used to drive the transducers. A typical transducer is a piezoelectric type requiring such high voltages in order to generate an adequately high energy beam. The transducer vibrates the foil on which it is mounted to agitate the cleaning fluid which is in direct contact with the foil. If a volatile and inflammable solvent were to be used as the cleaning liquid in such apparatus, the vapors therefrom could cause, if ignited by a pinhole arc, for example, an explosion or fire. It is because of this eventuality that cleaning apparatus utilizing electrically energized transducers do not use explosive type solvents. The use of such solvents in this art is highly desirable but is proscribed by such unpredictable, unsafe and hazardous conditions.
Accordingly, there is a need in the art to provide a megasonic cleaning apparatus of the type described that is inherently safe from such eventualities when used with volatile or inflammable solvents.